1. Field of the Invention
The present invention relates to an optical transmission/reception device for use in polarization multiplexing optical communication and an optical communication system using the device.
2. Description of the Related Art
In recent years, as computerization rapidly increases, data communication traffic rapidly increases in both backbone communication networks and consumer apparatuses and terminals. Therefore, there is a demand for a higher communication rate and a larger communication capacity. To meet the demand, an optical communication technique that is conventionally employed in backbone communication networks and in which a device for converting an electrical signal into an optical signal or vice versa, such as a semiconductor laser (light emitting device), a photodiode (light receiving device) or the like, is used, is becoming widespread in consumer apparatuses that conventionally employ a low-rate communication technique using electrical signals. However, consumer apparatuses or the like require a reduction in cost more strongly than backbone communication networks or the like do. The use of a very high-rate light emitting device or light receiving device for a single channel is not suitable for consumer apparatuses, since those devices are considerably expensive and an advanced peripheral system is also required. Therefore, a device is contemplated that has a relatively low rate as it is used singly, but has an increased communication capacity due to multiplexing of wavelengths, polarizations or the like. Particularly, a polarization multiplexing technique that allows optical signals having different polarized states to be simultaneously transmitted through a single optical fiber, is expected as an inexpensive and high-rate communication means since it can cause the number of channels to increase with an increase in the number of polarized states, i.e., the communication capacity can be increased.
FIG. 14 is a diagram showing an exemplary conventional optical communication system that is disclosed as means for generating a polarization multiplexed optical signal in Japanese Unexamined Patent Application Publication No. 2003-124913. In FIG. 14, double-headed arrows indicate polarization directions.
As shown in FIG. 14, horizontally-polarized waves 101 and vertically-polarized waves 102 that are output from two laser devices are transmitted through two polarization maintaining optical fibers 104A and 104B, respectively, that are connected to an input side of a polarization multiplexer 100 in a manner that allows the two types of polarized waves to enter in directions different by 90 degrees from each other. A signal that is output from an output side of the polarization multiplexer 100 to a polarization maintaining optical fiber 104C is orthogonal polarization multiplexed light 103 including optical signals having two polarized states where two types of polarized waves are oriented in directions different by 90 degrees from each other.